In many instances, and particularly for short-run printed graphics, offset lithographic printing has been supplanted by digital printing. Digital printing involves producing an image by localized control of ink distribution, rather than by the use of a mechanically or chemically altered plate. An advantage of digital printing systems is each print can differ, as no mechanical plate is necessary to transfer the image to the target substrate.
Digital printing encompasses various implementations of inkjet printing, but may also include dye sublimation printing, dry or liquid electrostatic printing, or any other method where the colorants are disposed within the printer from information provided by a rasterized digital image.
Digital printers, and particularly those which include a flatbed to support their printable material, and furthermore which cure their inks by ultraviolet (UV) radiation, are of particular interest to the lenticular trade. UV inkjet printers can print directly to plastic materials such as those used in the making of lenticular arrays. UV inkjet printers can often also reverse print process colors and then overlay the process colors with opaque white, so that a complete lenticular print may be devised in a continuous operation. Effective lenticular printing requires that at least two narrow bands be printed behind and parallel each lens on the flat back side of the lenticular lens sheet. When viewed from the side having the lens relief, the collective effect of the magnified image bands produces an image that changes with the angle of view.
The challenge in aligning a preformed linear lens sheet, such as those commonly prepared for lenticular printing, with any printing device is well known in the trade. Various optical and mechanical methods have been proposed. In a typical optical approach, a printed pattern is observed though the lenses and a positional adjustment is made, either by a human observer or by automation.
In the mechanical approach, a relative state of registration is attained between the structural element of a printer with a known position, such as a roller or table, and the lenticular lens material. The present invention predominantly belongs to the latter class, however, it is understood that visual effects can also be observed that verify correct alignment.
In traditional practice, a lenticular lens is provided by the manufacturer with a precisely cut reference edge. At the printing facility, the reference edge is made to bear against a raised ruler or fence, so that the location of the lens sheet is reliably established as it enters a printing device, such as an offset printing press.
However, the use of a single outer edge as a reference is not ideal, for a number of reasons. First, the reference edge must cut with great precision and consistency, or else the lenticular images, when printed and viewed, will have an irregular and inconsistent appearance.
Also, inconsistencies in lens pitch may arise from imperfect tooling of the lenticular embossing cylinder, variation in the speed and temperature of the extrusion line, environmental factors such as temperature and humidity, or the conditions of its storage. If a single reference edge is used, these errors will be expressed in a cumulative and summary way across the sheet. As a result, it is often the case that the effective center of the finished lenticular image appears to vary from print to print.
It may be appreciated by the foregoing discussion that the optimal case would be one in which the lens material was by some means aligned at its centerline with the printing device. In this circumstance, the registration is optimized at the center of attention, while any variation in the pitch is distributed to either side. Any decorrelation between the lens and the printer is therefore essentially reduced by half, and moved to the periphery of the printed sheet.
There are instances in the Prior Art where such a structural alignment is made between a printer component and the lenticular sheet. In U.S. Pat. No. 3,678,833 to Leach, two ridges on a guide roll fit into preformed channels on a compatibly made lenticular sheet. U.S. Pat. No. 5,764,266 to Azuma et al., fluted or toothed cylinders are used advance the lenticular sheet under a printhead. In U.S. Pat. No. 6,276,269 to Bravanec, a grooved cylinder is used to fix the location of the lens sheet as it passes through a printer. U.S. Pat. No. 6,460,993 to Pilu employs a similar grooved arrangement. In U.S. Pat. No. 9,021,947 Landa, a sheet of lens material having the same pitch as the sheet to be printed is attached to the roller, such that the relief faces of the two lenticular sheets may be held in a meshed state during printing. US20020187215 Trapani proposes fluted platforms so that multiple pieces of lenticular material may be set out in a tray.
However, these solutions are generally adapted to a fixed lens pitch, and cannot be readily adapted to the wide range of available lenticular materials, which can vary from 2.5 mm (˜10 lenses per inch) to 0.085 mm (˜300 lenses per inch).